1. Field of the Invention
The present invention relates to an apparatus and method for evaluating an organic electroluminescence display (hereinafter referred to as an xe2x80x9corganic EL displayxe2x80x9d) that makes use of an organic substance for its light-emitting substance, and more particularly relates to an apparatus and method for evaluating an organic EL display used in any of various types of display devices, such as the display panels of cellular telephones, the display panels of car audio systems, display panels for still or moving pictures, and the image displays of digital cameras.
2. Description of the Related Art
Organic electroluminescence elements (hereinafter referred to as xe2x80x9corganic EL elementsxe2x80x9d) have been the subject of considerable research and practical application in recent years.
FIG. 5 is an enlarged cross section of the main components of a conventional type of organic EL element 1. This organic EL element 1 comprises a glass substrate 2, an anode 3, a hole transport layer 4, an electron transport light-emitting layer 5 and a cathode 6. Direct current is supplied by applying a specific voltage between the anode 3 and the cathode 6 from a DC power supply 7.
A transparent electrode made of ITO (Indium Tin Oxide) or the like is employed for the anode 3, a diamine dielectric (TPAC) is employed for the hole transport layer 4, an aluminum complex (Alq) is employed for the electron transport light-emitting layer 5, and the carrier rebonding rate is raised by laminating materials with different carrier transport capabilities. Magnesium (Mg), aluminum (Al), or the like is employed for the cathode 6.
With an organic EL element 1 structured such as this, the carriers (hole and electron charges) injected from the anode 3 and the cathode 6 are confined in the organic layer of the electron transport light-emitting layer 5, the carrier rebonding efficiency rises sharply, and a high level of brightness (over 1000 cd/m2) can be obtained at a voltage of 10 volts or less.
Such elements are therefore expected to find use in cellular telephones, car audio systems, household electronics, and so forth.
FIG. 6 is a circuit diagram illustrating one pixel 11 in an active matrix type of organic EL display 10. The organic EL display 10 comprises a plurality of selection lines VG (scanning lines) and signal lines 9VD) arrange din a matrix, with the pixel 11 connected at the intersection of these lines.
The pixel 11 comprises a switching circuit 12, a constant current circuit 13, and an organic EL pixel 14 constituted by the above-mentioned organic EL element 1. The organic EL pixel 14 emits light when supplied with a constant current by the application of a fairly constant specific voltage from a voltage supply line VLC to the constant current circuit 13.
The pixel 11 has been disclosed in Japanese Laid-Open Patent Application H5-107561 and elsewhere: for example as shown in FIG. 7, a first transistor 15 consisting of a thin film transistor (TFT) or the like is employed as the switching circuit 12, and a second transistor 16, similarly made of TFT or the like, and a capacitor 17 are employed as the constant current circuit 13.
The first transistor 15 is switched in order to supply a constant current to the organic EL pixel 14.
The second transistor 16 is switched by the first transistor and is connected to the organic EL pixel 14.
The capacitor 17 is selected to help supply a constant current to the organic EL pixel 14 according to the specific discharge time thereof.
With a pixel 11 structured such as this, the selection of the pixel 11 is made by the first transistor 15, the result of the selection is transmitted to the second transistor 16, the voltage applied to the pixel 11 is controlled by the second transistor 16 and by the capacitor 17, which is able to hold a specific electrical charge for a specific length of time, and a fairly constant specific voltage from the voltage supply line VLC is maintained, thereby reducing the difference in voltage between the various pixels 11.
In order to evaluate an organic EL display 10 structured such as this, in the past the organic EL display 10 was actually driven only after a drive circuit (not shown) was attached to the organic EL display 10 and everything put together in a form similar to that of an actual finished product, and the work of detecting line defects or dot defects was performed by separate image evaluation devices.
Therefore, a problem was that discrepancies occurred between the various evaluation devices or in the evaluation standards, and this led to lower detection accuracy.
Another method is for the drive or luminescence state of the organic EL display 10 to be visually evaluated by a human, but a problem was that there was variance in the evaluation results depending on the experience of the evaluator and how well he or she was performing on a given day.
Furthermore, if an element is decided to be defective as a result of evaluation, the organic EL display 10 ends up being discarded along with the above-mentioned drive circuit parts attached to it, which is a problem in that it is wasteful. This also results in a waste of the time spent in evaluation.
Organic EL elements have been disclosed in the above-mentioned Japanese Laid-Open Patent Application H5-107561, as well as in Japanese Laid-Open Patent Applications H9-260061 and H10-321367 and elsewhere.
The present invention was conceived in light of the above problems, and it is an object thereof to provide an apparatus and method for evaluating an organic EL display, with which the drive circuit used for testing the organic EL display has a simple circuit configuration, and which yields evaluation results of high reliability.
It is another object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which the detection accuracy is high and it is possible to evaluate the organic EL display itself, before the finished product drive circuits have been incorporated into the organic EL display.
It is another object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which pixel defects of organic EL display elements can be detected by efficiently detecting, with a simple circuit configuration, the micro-current flowing to the organic EL elements.
It is another object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which the micro-current can be efficiently detected so that the drive current supplied to the organic EL elements for the purpose of detection does not become superposed between a number of organic EL elements.
It is yet another object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which it is possible to suppress the decrease in yield caused by dealing with defective products due to the evaluation results.
Specifically, according to the present invention it is possible to test an active-matrix type organic EL display after assembly of the pixels, the signal lines, the selection lines, and the voltage supply lines by energizing each pixel in turn, measuring a first steady-state current flowing to the EL element after the associated capacitor has been charged through the first transistor, and the second transistor is switched on, by measuring a second steady-state current after the capacitor has been discharged and the second transistor is switched off, by comparing the two measured currents, and by identifying a pixel as defective or defect-free based on the difference between the two currents.
The first aspect of the invention is an apparatus for evaluating an organic EL display having organic EL elements as pixels, wherein the the first and second currents as defined above are measured for each pixel constituted by an organic EL element and pixel defects are identified by detecting a difference in the current values thereof.
The above organic EL display can have a constant current circuit for driving the organic EL elements, and a switch for switching the voltage in order to make the constant current produced by this constant current circuit variable.
The above organic EL display can have a constant current circuit such as TFT for driving the organic EL elements, and signal lines and selection lines for selecting the organic EL elements.
There can be provided signal lines and selection lines for selecting the organic EL elements, and the drive current and discharge current values can be measured by switching either the signal line or the selection line for each pixel constituted by an organic EL element.
There can be provided signal lines and selection lines for selecting the organic EL elements, and the signal line or the selection line can be switched for each pixel constituted by an organic EL element so that drive current is supplied to the organic EL elements and the charge stored in the organic EL elements is discharged.
There can be provided a capacitor for supplying a constant current to the organic EL elements, drive voltage can be supplied to each pixel constituted by an organic EL element, and the charge stored in the capacitor can be discharged.
There can be provided a first transistor that performs switching for supplying a constant current to the organic EL elements, and a second transistor that is switched by the first transistor and is connected to the organic EL elements, and the first transistor can be switched for each pixel constituted by an organic EL element, whereby the drive current is supplied to the organic EL elements over a first specific time, and the second transistor is kept in a non-conducting state over a second specific time following this first specific time.
There can be provided signal lines and selection lines for selecting the organic EL elements, and voltage supply lines for supplying voltage to the organic EL elements, the signal line or the selection line can be switched for each pixel constituted by an organic EL element, in a state in which this voltage supply line is ON, and the drive current and discharge current flowing to the organic EL elements can be measured.
There can be provided signal lines and selection lines for selecting the organic EL elements, the signal line or the selection line can be switched for each pixel constituted by an organic EL element, and the current supplied to the organic EL elements can be controlled and the first current and the second current flowing to the organic LE elements as defined above measured.
The second aspect of the invention is an apparatus for evaluating an organic EL display, especially applicable to an active matrix type thereof, having organic EL elements as pixels, signal lines and selection lines for selecting the organic EL elements, and voltage supply lines for supplying voltage to the organic EL elements, said evaluation apparatus having a detection or test voltage generation circuit that generates a detection voltage to the signal lines, selection lines, and voltage supply lines, a control signal generation circuit that generates a control signal for sequentially applying this detection voltage at a specific period to the signal lines, selection lines, and voltage supply lines, a connection switching circuit for connecting this control signal to the organic EL elements via the signal lines, selection lines, and voltage supply lines, a current detection circuit for detecting the first current and the second current flowing to the organic EL elements as defined above, and a detected decision circuit that decides whether the organic EL elements are defective or non-defective based on the detected current values.
The third aspect of the invention is an apparatus for evaluating an organic EL display having signal lines and selection lines arranged in a matrix, and organic EL elements as pixels connected to said signal lines and selection lines at the intersections between these signal lines and selection lines, wherein the signal line or the selection line is switched for each pixel constituted by an organic EL element, that organic EL element is energized, and the first and second current values as defined above are measured for said organic EL elements, and pixel defects are detected by detecting a difference in the current values thereof.
The fourth aspect of the invention is an apparatus for evaluating an organic EL display having signal lines and selection lines arranged in a matrix, and organic EL elements as pixels connected to said signal lines and selection lines at the intersections between these signal lines and selection lines, wherein the signal line or the selection line is switched for each pixel constituted by an organic EL element, and that organic EL element is energized, a first sampling is performed for the first current values as defined above, and a second sampling is performed for the second current current values as defined above, whereby the first and second current values are measured, and pixel defects are detected by detecting a difference in the current values thereof.
The fifth aspect of the invention is a method for evaluating an organic EL display having organic EL elements as pixels, wherein the first and second current values as defined above are measured for each pixel constituted by an organic EL element, and pixel defects are detected by detecting a difference in the current values thereof.
With the apparatus and method of the present invention for evaluating an organic EL display, the drive (i.e., the testing) of an element is performed after the discharge of the previous element after the supply of drive current to the various pixels (organic EL elements) of the organic EL display; that is, the difference is measured between the drive current and discharge current values of the organic EL elements, so the supply and discharge of drive current can be performed for each pixel (organic EL element), and the organic EL elements can be tested one at a time.
If the difference between the current values is under the specified level, it can be concluded that the organic EL element that constitutes a pixel is operating normally.
With the first aspect of the invention in particular, any difference between the drive current and discharge current values is detected for each pixel constituted by an organic EL element, so the next pixel (organic EL element) is always tested in the same way upon completion of the discharge of the previous pixel, the drive current value resulting from the previous detection does not remain in the next pixel, and successive evaluations can be carried out for all of the pixels in a reliable manner.
With the second invention in particular, there are provided a detection voltage generation circuit, a control signal generation circuit, a connection switching circuit for connecting to the organic EL display, a current detection circuit, and a defect decision circuit, so unlike with a conventional evaluation apparatus, in which a drive circuit was attached to the organic EL display and everything put together in a form similar to that of an actual finished product, the evaluation work can be carried out for just the organic EL display.
With the third aspect of the invention in particular, the signal line or the selection line is switched for each pixel constituted by an organic EL element connected at the various intersection between these signal lines and selection lines arranged in a matrix, and the difference between the first and second currents as defined above is detected, so each pixel can be evaluated quickly by selecting a signal line or selection line.
With the fourth aspect of the invention in particular, a first sampling is performed for the first current values within the drive time of the organic EL elements, and a second sampling is performed for the second current values at the end of the discharge time following this drive time, so it is possible to measure a current value that is suitable for the evaluation of each organic EL element.
With the fifth aspect of the invention in particular, just as with the first invention, detection is carried out for a given pixel (organic EL element) after completion of discharge of the previous pixel, so the drive current value resulting from the previous detection does not remain in the next pixel, and successive evaluations can be carried out for all of the pixels in a reliable manner.